flvmux: Always put a duration tag in the metadata

flvmux: Always put a duration tag in the metadata

Some Flash players (for instance JW Player) always expect a duration
tag, otherwise they don't start playback.

If duration can be queried from the sink pads or is provided as a tag,
use it. Otherwise try to determine it from the last seen timestamp of
the sink pads after EOS and rewrite it in the header before writing
the index.